The invention concerns an assembly of an end-fitting, having an axially extending through opening, with a flexible pressure pipe which is of a non-bonded structure comprising a number of layers including an inner carcass of an interlocking structure made from metallic strips forming a screw thread, the end part of which extends, in the assembled condition, into the through opening.
Flexible pressure pipes are used in a variety of applications, including water supply lines, sewage lines and lines for transporting chemicals such as liquid ammonia and phosphoric acid and also high pressure offshore flexible pipes for the oil and gas industry.
The joint between the carcass and the end-fitting must be able to resist the axial tensile load acting on the carcass, when the flexible pipe is descending over a great length. As the carcass is of an interlocking structure of a type which allows a mutual screwing movement of the windings of the metallic strips, the joint moreover must prevent the carcass from becoming unwound.
Conventionally, such a joint is provided by screw shaped ribs which, during the extrusion of an inner lining of a plastic material around the carcass, are pressed into the grooves between the windings such that the carcass is secured by the lining which again is secured by the end-fitting. Owing to the notch sensitivity of the lining caused by this construction, the lining is, however, inclined to crack.
This tendency is especially dangerous because temperature stresses can arise in the joint between the lining and the carcass, because the plastic material of the lining has a much higher coefficient of expansion than the material of the carcass, which could be of e.g. stainless steel.
The object of the invention is to provide an assembly of the type mentioned in the opening paragraph in which the joint between the carcass and the end-fitting is a simple and cheap structure and provides a greater resistance to axial tensile loads acting on the carcass and against unwinding of the carcass than has been known before.
This is achieved in that the assembly comprises an annular holding groove which is formed in the wall of the through opening, and a lock nut which is screwed on the screwthread-formed strips of the end part of the carcass and fitted into the holding groove. In this construction the joint between the carcass and the end-fitting is independent of the inner lining which moreover can be made with optimal strength. When fixed in the holding groove, the lock nut also prevents the carcass from becoming unwound.
By interposing a galvanic insulation material having a high electrical resistance between the lock nut and the end-fitting, an electric potential, which could cause galvanic corrosion, can be prevented from arising between said parts.
The galvanic insulation material may advantageously be PTFE, preferably containing some solid insulation material for improving the pressure strength and the form stability of the material.
In a simple and cheap embodiment, the galvanic insulation material can be in the form of an annular disc placed on each side of the lock nut. In a particularly expedient embodiment, the lock nut can also be coated with the galvanic insulation material or totally manufactured from it.
The lock nut itself can be locked to the carcass by e.g. welding which, however, is time consuming and strenuous work and involves the risk that some of the plastic material of the pipe may be harmed by the heat generated by the welding process.
The lock nut can, however, in a very simple way be locked to the carcass by screwing a screw radially into the lock nut and against the carcass. This screw can advantageously be used to rotate the lock nut and thereby tighten the carcass when the windings of this have been to some extent unwound.
For fixing the lock nut firmly in the holding groove of the end-fitting, the lock nut can be embedded in a casting material, e.g. epoxy.
The invention also concerns a method for assembling an end-fitting, having an axially extending through opening, with a flexible pressure pipe which is of a non-bonded structure comprising of a number of layers including an inner carcass of an interlocking structure made from metallic strips forming a screw thread, an end part of which extends, in assembled condition, into the through opening. The method comprises removing the layers surrounding the carcass at the end of the pipe, screwing a lock nut on the screwthread-form strips of the end part of the carcass, and placing the lock nut in a holding groove formed in the wall of the through opening. This method can be performed in a quick and time efficient manner in situ.